The 2 Stroke Crosshead
• The 2 stroke diesel crosshead engine works on exactly
the same principle and cycle as the 2 stroke diesel trunk
• The disadvantages of the two stroke diesel trunk piston
engine are that although it has a low overall height,
lubricating oil splashed up from the crankcase to
lubricate the liner can find its way into the scavenge
space, causing fouling and a risk of a scavenge fire.
There is also the likelihood of liner and piston skirt wear,
allowing air into the crankcase. This can supply the
required oxygen for a crankcase explosion should a hot
spot develop. The crankcase oil must have additives
which can cope with contamination from products of
combustion, and the acids formed during combustion
due to the sulphur in the fuel.
• The majority of 2 stroke
engines encountered at
sea are of the "crosshead"
type. In this type of engine
the combustion space
(formed by the cylinder
liner, piston and cylinder
head), and the scavenge
space are separated from
the crankcase by the
• The piston rod is bolted to the piston and passes
through a stuffing box mounted in the diaphragm
plate. The stuffing box provides a seal between
the two spaces, stopping oil from being carried
up to the scavenge space, and scavenge air
leaking into the crankcase.
• The foot of the piston rod is bolted to the
crosshead pin. The top end of the connecting
rod swings about the cosshead pin, as the
downward load from the expanding gas applies
a turning force to the crankshaft.
• To ensure that the crosshead reciprocates in alignment with the
piston in the cylinder, guide shoes are attached either side of the
crosshead pin. These shoes are lined with white metal, a bearing
material and they reciprocate against the crosshead guides, which
are bolted to the frame of the engine. The crosshead guides are
located inbetween each cylinder.
• Using the crosshead design of engine allows engines to be built with
very long strokes - which means the engine can burn a greater
quantity of fuel/stroke and develop more power. The fuel used can
be of a lower grade than that used in a trunk piston engine, with a
higher sulphur content, whilst high alkalinity cylinder oils with a
different specification to that of the crankcase oil are used to
lubricate the cylinder liner and piston rings and combat the effects of
• The most powerful diesel engines in the
world are two stroke crosshead engines.
Some of these engines have cylinder
bores approaching 1metre with a stroke of
over 2.5 metres. The crankshaft can weigh
over 300 tons, with the engine weighing in
excess of 2000 tons.